fixup! [LibOS] Update syscall fchmodat(), mmap(), readv() error codes to align with man page

Signed-off-by: Erica Fu <[email protected]>

Author: efu39

libos/src/sys/libos_wrappers.c

@@ -16,9 +16,11 @@
  * notably affects pipes. */
 
 long libos_syscall_readv(unsigned long fd, struct iovec* vec, unsigned long vlen) {
-    size_t arr_size;
-    if ((int)fd < 0)
+    struct libos_handle* hdl = get_fd_handle(fd, NULL, NULL);
+    if (!hdl)
         return -EBADF;
+
+    size_t arr_size;
     if (__builtin_mul_overflow(sizeof(*vec), vlen, &arr_size))
         return -EINVAL;
     if (!is_user_memory_readable(vec, arr_size))
@@ -33,10 +35,6 @@ long libos_syscall_readv(unsigned long fd, struct iovec* vec, unsigned long vlen
         }
     }
 
-    struct libos_handle* hdl = get_fd_handle(fd, NULL, NULL);
-    if (!hdl)
-        return -EBADF;
-
     maybe_lock_pos_handle(hdl);
 
     int ret = 0;